For any parachute system, it is important to predict the opening forces it will experience in order to make a safe and economic choice of materials to be used. Only limited data for these decisions is available and any numerical model sought to be used is also limited by the lack of data to use and to verify the model. Novel techniques are therefore needed to determine the structural behavior of a parachute during inflation. A different method for experimental measurement of parachute behavior during opening is also needed. There are other fabrics and textiles that are subjected to stress, such as, for example, balloons and the like.
The schematic of the standard quasi-static circular canopy parachute is shown in FIG. 1. This parachute comprises two main parts: the canopy and the suspension lines. A parachute drop generally consists of three principal stages: deployment, inflation, and descent. The deployment phase begins with the ejection of the payload from the aircraft, rocket or the like, and ends when the suspension lines and folded canopy have been fully extracted from the deployment bag. The full extension of the parachute system is marked by the "snatch" force impulse, an acquisition event that occurs when the falling payload accelerates the parachute mass up to its own velocity.
In most military airdrops, the deployment bag is attached to the aircraft by a static tether; hence, the time lag between payload ejection and the snatch point is usually small. In free-fall personnel drops and sport parachute jumps, the time delay is usually longer. In either case, the parachute shape at the end of the deployment phase is essentially that of an elongated but deflated tube. During the subsequent inflation phase, the elongated parachute transforms from a closed tube to an open canopy, ultimately increasing the aerodynamic drag and decelerating the payload. Eventually, a steady-state condition or descent is reached where the aerodynamic drag balances gravity and the payload drifts to earth at a relatively constant velocity.
A typical plot of the force on the payload versus time during parachute opening is shown in FIG. 2. As this figure indicates, the force on the payload, with the exception of the snatch impulse is small during the initial stages of inflation. As the inflation continues, the opening force exerted by the continually filling canopy increases to a peak, then decreases over time.
In the design of parachute systems, therefore, it is very important to use structural properties that have been developed under the representative force rates expected in flights. Without such data, the designer is potentially forced to incorporate unrealistic safety margins, resulting in a parachute that is heavier and costlier than necessary. Laboratory test data has generally been limited to that which can be acquired at quasi-steady strain rates. Past work has suggested that the properties of textile materials obtained through the typical quai-static testing process are inapplicable to dynamic strain conditions.
Past reported work presents the use of electronic strain gauge equipment for measuring stresses under various loading conditions. Testing materials at representative high strain rates of dynamic forces requires much more sophisticated equipment than found in a typical material testing laboratory.
It would be a great advantage in the art to have an improved methodology and apparatus, which takes advantage improved data acquistion techniques, such as that of advances in fiber optic sensors for fabrics, textiles and other flexible structures.
Still another advantage would be to use fiber optics technology to determine the stress/strain relation of flexible devices such as parachutes during deployment and inflation.
It is therefore an object of this invention to provide a method and apparatus for high speed data acquisition devices as well as electronic RF (radio frequency) transmitter receivers are used for signal processing and transmission of the information to a ground station.
Another object is to provide a method and apparatus using fiber optics technology to evaluate parachutes under load.
Yet another object is to obtain a means of determining the dynamic properties of structural materials in-situ and in real-time.
Other objects will appear hereinafter.